OXYGEN CONSUMPTION AND THERMOREGULATION Metabolic Rate Requirements for determining the basal metabolic rate include a 12-h fast and full relaxation. These conditions are nearly impossible to fulfill in the neonate, however, the maximum increase in energy metabolism is less than 10% after a feeding. Therefore, measurements in neonates are usually made in the fed state over a 5-10 minute period, although a 1-hour period is preferred to ensure that the minimum oxygen uptake has been reached at some point during the recording period. Measurements should be made in a thermoneutral environment and the neonate may be sleeping during these measurements. The minimum metabolic rate measured in this way is called the standard metabolic rate (SMR). The SMR, although proportional to body size, is not directly proportional. Smaller animals will have a higher metabolic rate per unit of body mass; this is described by the law of metabolic reduction (SMR = 3.29 x W0.75). Although this is useful for predicting differences in metabolic rates between species, it has limitations when comparing individuals within a species. Sex, body shape and body composition all affect metabolic rate, however, age is the biggest factor preventing a single equation from being useful within a species. The SMR is lower than predicted in newborns, with a 3-kg human having a lower SMR than a 3-kg mature rabbit. The most appropriate exponent during this period is 1, meaning that the SMR per unit of body mass is nearly independent of weight, whereas a decease would normally be expected. However, from 5-20 kg, human SMR is greater than in mature animals of the same weight range. Above 20 kg, SMR finally conforms to the law of metabolic reduction. Values for the first few hours of life in infants are within the range of 4.6 to 5 ml/kgmin. The postnatal increase appears to be species-dependent; it takes a matter of days in primates and laboratory animals and a matter of hours in the lamb. The more rapid increase in SMR does not occur in fasted lambs, however, suggesting that the onset of digestion and absorptive processes may account for much of this increase. Much of the “violation” of the law of metabolic reduction in neonates may be related to differences in extracellular fluid content of neonates. Extracellular fluid may account for 44% of a 4000-g full-term neonate and up to 58% in premature infants (1000-g) while only accounting for 16% of an adult human. Since ECF is not metabolically active, it may be more accurate to refer to active tissue mass, or to active tissue mass plus 16% to bring values to an adult basis. Although this manipulation does bring neonatal values back into line with the law of metabolic reduction, deviations seen at older ages are not so easily explained. The higher rates seen at these ages may be a function of higher thyroid function at these ages and especially as a function of brown fat metabolism, which can account for as much as 30% of the SMR at this age. Heat Production Thermoregulation in homeothermic organisms, especially animals that have a small surface:volume ratio and an effective insulating layer of subcutaneous fat or hair, is primarily concerned with dissipating metabolic heat. In contrast, small animals or animals without an effective insulating layer need effective mechanisms for supplemental heat production to ensure survival. The thermoneutral zone in neonates is defined as the range of ambient temperature within which metabolic rate is at a minimum, and within which temperature regulation is achieved by nonevaporative physical processes alone. In the unclothed resting adult, the lower limit of the thermoneutral zone is 26-28 C, while in newborns it is 32-34 C. Environmental temperature conditions that require no thermoregulatory effort in the adult may easily overtax the metabolic thermoregulatory capabilities of the newborn infant. In very small premature infants, the lower limit of the thermoneutral zone may be as high as 35 C. There are three principal modes of heat production in response to cold stress: 1) voluntary muscle activity, 2) involuntary tonic or rhythmic muscle activity, which may be low-intensity and not visible, or higher intensity visible shivering, and 3) non-shivering thermogenesis. In most mature mammals, shivering is quantitatively the most important involuntary mechanism for thermoregulation and non-shivering thermogenesis develops only after long-term cold exposure. In neonates, non-shivering thermogenesis is a quantitatively important and effective mechanism for heat production. The elicitation of non-shivering thermogenesis is mediated by the sympathetic nervous system and can be blocked by -receptor antagonists such as propranolol. Non-shivering thermogenesis is the preferred mode of heat production in neonates and will suppress shivering. Metabolically active brown fat is a major site of thermogenesis, and comprises approximately 1 per cent of the human adult body mass, while the metabolically inactive white fat functions primarily as fat storage. In neonates, it may comprise as much as 5 to 7% of body weight (except in the pig, which completely lacks brown fat). Brown fat cells contain an abundance of sympathetic nerve fibers which actually maintain synaptic contact with the cell membrane. These release norepinephrine which triggers thermogenesis and activates lipase. The cells have a centrally located nucleus surrounded by multiple fat lobules surrounded by mitochondria. The specialized function depends on a 32 KDa protein in the mitochondrial inner membrane called the “uncoupling protein” or thermogenin. The role for this protein is to shortcircuit the electrochemical gradient generated by the respiratory chain. As a general rule, except in brown adipose tissue, coupling between substrate oxidation and ADP phosphorylation in the mitochondria is due to the development of an electrochemical gradient on either side of the mitochondrial matrix. This gradient cannot be discharged except at specific sites (F1-ATPase) where ATP-synthase is located. This controlled discharge of this gradient generates the energy used for ATP production. Thus electron transfer from substrates to oxygen produces stochiometric amounts of ATP during coupled oxidative phosphorylation. By contrast, in brown adipose tissue mitochondria, the uncoupling protein allows discharge of this gradient without generation of ATP. The energy generated by uncoupling oxidative phosphorylation is simply released as heat. Postnatal development of respiratory enzymes and the uncoupling protein (UCP) in brown adipose tissue occurs during the first few hours after birth and is accelerated and enhanced by cold stress. The synthesis of the uncoupling protein is under noradrenergic control, and the UCP gene is acutely regulated at the level of transcription after activation of the plasma membrane -adrenoreceptors of the brown adipocyte. Therefore, the rapid increase in UCP found in brown adipose tissue mitochondria of cold-stressed neonates is most likely due to a rapid increase in the rates of transcription of the UCP gene. Blood flow through intrascapular deposits of brown fat in the newborn rabbit increase from approximately 90 ml/100 gmin to over 700 ml/100 gmin during cold exposure, accounting for up to 25% of cardiac output. Brown adipose tissue can provide up to 2/3 of the total heat provided from non-shivering thermogenesis under conditions of maximal stimulation. The existence of extensive brown fat deposits at birth may be a compensatory mechanism for the smaller body size since thermoregulatory efficiency of non-shivering thermogenesis is greater than that of shivering. In precocial newborns, the extent of non-shivering thermogenesis is greatest at birth and vanishes within a few weeks. Cold exposure during this period prevents the disappearance of non-shivering thermogenesis, however, with increasing age, the extent to which it can be maintained (or evoked) is decreased. Altricial neonates have gradual increases in non-shivering through the first few weeks of life. Cold-induced heat production maintains body temperature as long as heat loss does not exceed the capability for heat production. As soon as this capability is exceeded, body temperature begins to decrease. Both cold-induced heat production and basal heat production will decrease as body temperature falls, with oxygen uptake decreasing by a factor of 2- to 3-fold per 10 C change in body temperature. The thermoregulatory drive as generated in the thermointegrative area of the CNS is reduced with increasing hypothermia. Neonates of all species, except the ground squirrel and the golden hamster, are capable of a cold-induced metabolic response immediately after birth, however, during the first few hours or days of life, this increase is reduced. The lamb is the only species of those studied that had a response that was actually greater than that predicted to be maximal; lambs were also the only precocial species studied and this data would be consistent with the changes in non-shivering thermogenesis described earlier.